Cancer spreads — or metastasizes — when cancer cells break away from the original tumor and travel through your bloodstream or lymphatic system to other parts of your body.

This video shows how a type of skin cancer called melanoma can metastasize. Although the appearance of cancer cells and the way they spread varies depending on the type of cancer, the general concepts shown here are useful in understanding how any cancer can spread. Keep in mind that not all cancers metastasize.

Melanoma begins in the melanocytes — cells that produce skin color, or pigment. Melanoma typically begins as a mole. If you catch the melanoma early and it hasn't spread, it can be surgically treated. Here you see a superficial melanoma being removed, along with the surrounding skin. The extra skin is taken to ensure that no cancer cells are left behind.

Melanoma is dangerous because it can spread beyond what you can see, moving deep into your skin where it can gain access to your lymphatic vessels. This allows cancer cells to travel to distant locations in your body. Cancer cells can also travel to different parts of your body by way of your blood vessels.

In this example, melanoma cells migrate to a lymph node. From the lymph node, the melanoma cells can then travel to other parts of your body. Here, you see cancer spreading to the lung.

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